Interpretive Summary: Haploids are plants with one-half of the normal number of chromosomes. The production of doubled haploid plants yields uniform inbred lines and is especially desirable as an alternative to sexual inbreeding of longer-generation crops. Onion (Allium cepa L.) is a biennial plant and amenable to the production of haploids from the female (gynogenic) gametes. Although onion haploids can be routinely generated from many populations, chromosome doubling to produce fertile inbred lines remains the main challenge to the successful deployment of this breeding strategy. In this study, we evaluated a variety of chemicals over different treatment periods to assess their efficacy to double the chromosome number of onion haploids. The best system is to treat onion haploid embryos with a two-day treatment in liquid media with 50 mM APM. APM is a compound called Amiprofos-methyl. It is a compound used to double the chromosome number of cells. Alternative treatments with lower but still acceptable rates of doubling were a two day treatment in liquid at 25 mM APM or a two day treatment on solid medium at 50 mM APM. These results will be of use to onion breeders in the private and public sectors interested in using haploidy to produce uniform onion inbreds.

Technical Abstract:
A novel approach for chromosome doubling consisting of treatments of embryos instead of parts of micropropagated plants was investigated. Based on two year trials, amiprofos-methyl (APM) was superior to oryzalin on the basis of lower toxicity and the range of concentrations of APM was narrowed. The addition of 2% DMSO and 1% Triton X-100 to 25 mM APM was found negative in all treatments. A final experiment with 6,658 embryos demonstrated that a two-day treatment in liquid media with 50 mM APM was the most successful for chromosome doubling. At this treatment 36.7% of plants were diploid but survival rate was reduced to 52.5% compared to non-treated control. A two days treatment in liquid at 25 mM APM or a two day treatment on solid medium at 50 mM APM resulting with 28.9% and 21.3% diploids respectively might be alternatives since compared to untreated control these two treatments reduced survival rate to only about 24%. Final ploidy and fertility of the large proportion of induced mixoploid plants (up to 30.3%) need to be evaluated in further studies.